(1) Field of the Invention
The present invention relates to a transfer type one-component magnetic developer for use in the electrostatic photography. More particularly, the present invention relates to a one-component dry magnetic developer suitable for use in the electrostatic photographic process in which an electrostatic latent image formed on a photosensitive plate and the developed image is transferred onto a plain paper as a transfer sheet to form a reproduced or printed image.
(2) Description of the Prior Art
As the developer capable of developing an electrostatic latent image without using a particular carrier, so-called one-component or carrierless magnetic developers comprising a finely divided magnetic material incorporated in developer particles are known and broadly used in the field of electrostatic photography.
As one type of such one-component magnetic developer, there is known a so-called conductive magnetic developer in which a finely divided magnetic material is incorporated in developer particles to impart a magnetically attractable property and a conducting agent such as conductive carbon black is distributed on the surfaces of the particles to impart an electric conductivity (see the specifications of U.S. Pat. No. 3,639,245 and No. 3,965,022). It is known that when this conductive magnetic developer is brought into contact in the form of a so-called magnetic brush with an electrostatic latent image-supporting plate to effect development of the latent image, an excellent visible image free of a so-called edge effect or fogging is obtained. However, it also is known that a serious problem arises when this image of the developer is transferred to an ordinary transfer sheet. More specifically, as disclosed in Japanese Patent Application Laid-Open Specification No. 117435/75, if the inherent electric resistance of a transfer sheet used is lower than 3.times.10.sup.13 .OMEGA.-cm as in case of a plain paper, broadening of the contour of the transferred image or reduction of the transfer efficiency is readily caused by scattering of developer particles. This defect may be moderated to some extent if a resin, wax or oil having a high electric resistance is coated on the toner-receiving surface of a transfer sheet, but this improving effect is relatively low under a high humidity condition. Furthermore, the cost of transfer sheets is increased by coating with a resin or the like, and the touch or feel is reduced by the presence of such coating.
As another type of the one-component magnetic developer, there is known a one-component non-conductive magnetic developer comprising particles of a homogeneous and intimate mixture of a finely divided magnetic material and an electricity-detecting binder. For example, the specification of U.S. Pat. No. 3,645,770 discloses an electrostatic photographic reproduction process in which a magnetic brush (layer) of such non-conductive magnetic developer is charged by corona discharged with a polarity opposite to the polarity of an electrostatic latent image to be developed, the charged developer is brought into contact with an electrostatic latent image-supporting plate to develop the latent image and the formed developer image is transferred onto a transfer sheet. This electrostatic photographic reproduction process is advantageous in that a transfer image can be formed even on a plain paper as a transfer sheet. However, this process is insufficient in some points. For example, it is difficult to uniformly charge the magnetic brush of the non-conductive magnetic developer entirely even to the deep root portion thereof, and it is ordinarily difficult to form an image having a sufficient density. Furthermore, since a corona discharge mechanism should be arranged in the zone of a developing device, the structure of the copying apparatus becomes complicated.
There have recently been proposed a process in which an electrostatic latent image is developed by utilizing charging of a non-conductive magnetic developer owing to frictional contact of the developer with the surface of an electrostatic latent image-supporting plate (see Japanese Patent Application Laid-Open Specification No. 62638/75) and a process in which development is carried out by utilizing dielectric polarization of a non-conductive magnetic developer (see Japanese Patent Application Laid-Open Specification No. 133026/76). In the former process, the development conditions should strictly be controlled, and if the development conditions are not strictly controlled, fogging is readily caused in a non-image area (occurrence of fogging is especially conspicuous when the degree of contact between the surface of a photosensitive material and the top ends of spikes of magnetic toner particles is high) or fixing or blocking of magnetic toner particles on a developing sleeve is caused. These defects become serious when the reproduction operation is continuously conducted. In the latter process, the problem of fogging does not arise, but since an electrostatic latent image is converted to a visible image by the developing charges produced by a dielectric polarizing effect induced by the magnetic toner, a low-potential portion of the latent image is not effectively developed. Therefore, a low-density portion of an original is not effectively reproduced and it is difficult to reproduce a half tone in a print.
Furthermore, copies obtained according to the above two processes lack a sharpness, and if a p-type photosensitive material such as selenium is used for a photosensitive plate and a positively charged image is developed, according to any of these two processes, it is difficult to form images having a sufficiently high density.